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Disorienting mosquitoes with a blend of odors

A combination of odorous chemicals can potentially disorient mosquitoes, …

As we enjoy the outdoors through various summer activities, we also have to deal with pests like mosquitoes, which are potential disease carriers, on top of being annoying. DEET (N,N-diethyl-m-toluamide) is an effective repellent, but it requires high concentrations, can be a skin irritant, and is damaging to some materials, including plastics. Certain species of mosquitoes are also developing resistance to DEET. There is a real need for new repellents, particularly in developing countries that are vulnerable to malaria and dengue.

To further this search, scientists are looking for ways to disrupt the ability of female mosquitoes to detect carbon dioxide (CO2), a gas that we exhale. Female mosquitoes use carbon dioxide as a cue for finding hosts. Once they detect a plume of the gas, they will fly upwind to find the source. In a recent issue of Nature, a paper by Stephanie Turner and her colleagues reports that a combination of odors could disrupt the mosquito’s response to carbon dioxide.

The researchers screened for chemicals that would affect behavioral response to carbon dioxide in female mosquitoes from three major disease-transmitting species: Anopheles gambiae, Culex quinquefasciatus, and A. aegypti. Turner found that 2,3-butanedione (a natural byproduct of fermentation that has a buttery flavor) activated the cpA neuron, which is a highly sensitive carbon dioxide detector. A brief exposure (a one-second pulse) causes the neurons to be significantly less sensitive to carbon dioxide for at least 5.5 minutes.

To further enhance the effect of 2,3-butanedione, the authors tested multiple combinations of chemical odors. They found that a combination of 2,3-butanedione, 1-hexanol (a volatile organic alcohol), 1-butanal (an acrid-smelling flammable liquid), and 1-pentanal (a food additive that has a woody, nutty, and fruity odor) was the most effective at making mosquitoes unresponsive to carbon dioxide.

Turner and her team tested this combination of four chemicals in semi-field experiments in Kenya. They placed two huts in a large enclosed greenhouse and equipped the huts with carbon dioxide traps for mosquitoes. Then the researchers dispensed the odor blend (one percent in paraffin oil) using small fans in one of the huts. In overnight experiments, Turner and her team released female mosquitoes (Culex quinquefasciatus) in the greenhouse and counted the number of mosquitoes entering the huts and traps.

They found that the odor blend treatment significantly reduced the number of mosquitoes entering the huts and flying into the carbon dioxide traps (about 40 percent in the untreated hut compared to roughly 20 percent in the treated hut).

In a separate corroborating experiment, Turner and her colleagues placed only one hut in a greenhouse. When the hut was treated with the odor blend (three percent in paraffin oil), only about 10 percent of the mosquitoes entered the hut and reached the carbon dioxide trap, while roughly 35 percent of the mosquitoes would reach the trap in the hut when there is no treatment.

The blend of the four odorous chemicals appears to disorient the mosquitoes, disrupting their ability to seek out carbon dioxide and fly towards the source. Dispensing a blend of odorous repellents has the benefit of being able to protect many people in a given area. You also wouldn’t need to apply this blend directly on your skin, but you would end up breathing it in.

Turner and her team also found that the odor blend does not diminish the effect of DEET on mosquitoes. Thus, the blend could potentially be used in conjunction with DEET. You would then have both short-range repellency from DEET and long-range repellency from the odor blend. However, the authors have not reported the maximum range of operation for a given amount of the chemical mixture.

One caveat is that the odorous chemicals are not safe for use around humans without further testing, as they could be problematic in high concentrations. However, the researchers have proved that the concept of using disorienting odors works for repelling mosquitoes. As with many drugs, the chemical structure of the odors could be modified so that they are still effective without being harmful to humans.

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Yun Xie
Yun Xie / Yun Xie is a contributing science writer at Ars, where she covers the latest advancements in science and technology for Ars. She currently works in scientific communications, policy, and review. Emailreenxie@gmail.com//Twitter@yun_xie